During the elongation phase of protein synthesis, elongation factor Tu (EF-Tu) delivers the cognate aminoacyl-tRNA (aa-tRNA) to the A-site of the ribosome. Mammalian mitochondrial EF-Tu (EF-Tumt) must execute this task using a set of mitochondria) aa-tRNAs that are non- canonical in that they lack many of the invariant or semi-invariant residues that stabilize the 3-dimensional structure of canonical tRNAs. While both E. coli EF-Tu and EF-Tumt successfully bind mammalian mitochondria) aa-tRNAs, bacterial EF-Tu is unable to deliver mitochondria) aa-tRNAs to the A-site of the ribosome. EF-Tu is folded into three domains. Domain II of EF-Tu contains the binding pockets for both the 3' acceptor stem and the 5' end of the aa-tRNA. The goal of the proposed research is to examine the roles of specific residues in Domain II on the ability of EF-Tumt to interact with mitochondria) aa-tRNAs. Site-directed mutagenesis of residues in EF-Tumt predicted to be important for its interaction with as tRNAs will be carried out. A set of complementary mutagenesis experiments will be performed with E. coli EF-Tu. The effects of these mutations on the structure and function of prokaryotic and mitochondria) EF-Tu will be examined in detail.